Quite recently, Mr. B. B. Woodward, F.G.S., of the British Museum (Natural History), called my attention, for the first time, to the fact that the name Plutonia, which I adopted for a genus of Trilobites in 1868, had previously been used by Stabile (Atti. Soc. Ital. Sci. Nat. vii, p. 121, 1864) for a genus of Mollusca. As Stabile's generic term has therefore a priority of four years it is necessary that I should rename the Trilobite, and it has been suggested to me by Mr. Belinfante, B.Sc., Assist. Sec. Geol. Soc., that Plutonides would be the most suitable term and the one least likely to lead to confusion. In the Report of the British Association for 1868, p. 69, where the genus is first mentioned, after describing the beds in which it occurs I refer to it as follows: “The new genus, for which the author proposes the name Plutonia, is only known to occur in these beds. This remarkable fossil is of very large size, equalling, indeed, in this respect Paradoxides Davidis. It is, perhaps, also more nearly allied to the genus Paradoxides than to any other known, but its peculiar character of being covered all over with very strong tubercles, associated with an unusual position for the eye suture, and straight, very long thoracic pleuræ, is sufficient to stamp it a new and distinct genus.”.